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Abstract:

A laser diode which realizes NFP with a stable and uniform shape. The
laser diode includes, on a semiconductor substrate, an active layer, one
or a plurality of strip-shaped current confinement structures confining a
current which is injected into the active layer, and a stacked structure
including one or a plurality of strip-shaped convex portions extending in
an extending direction of the current confinement structure.

Claims:

1. A laser diode comprising: an active layer; and a stacked structure
including one or a plurality of strip-shaped current confinement
structures confining a current which is injected into the active layer,
wherein the current confinement structure is in a ridge shape, and the
current confinement structure includes one or a plurality of strip-shaped
concave portions extending in an extending direction of the current
confinement structure, the concave portions disposed in an upper part of
the ridge shape.

2. The laser diode according to claim 1, wherein a width of the concave
portion is 2 μm or above, and 5 μm or below.

3. The laser diode according to claim 1, wherein a depth of the concave
portion is almost equal to or smaller than a height of the current
confinement structure.

4. The laser diode according to claim 1, wherein the concave portion is
formed in a central part in a width direction of the current confinement
structure.

5. The laser diode according to claim 4, wherein a plurality of the
concave portions are formed, and a space between the concave portions is
2 μm or above, and 5 μm or below.

6. The laser diode according to claim 1, wherein the concave portion is
formed in an area except the central part in the width direction of the
current confinement structure.

7. The laser diode according to claim 6, wherein a plurality of the
concave portions are formed and the concave portions are formed in both
of a pair of areas sandwiching the central part, a space between the
concave portion formed in one of the pair of areas sandwiching the
central part, and one of end edges in a width direction of an upper part
of the current confinement structure is 2 μm or above, and 5 μm or
below, and a space between the concave portion formed in the other of the
pair of areas sandwiching the central part, and the other of end edges in
a width direction of the upper part of the current confinement structure
is 2 μm or above, and 5 μm or below.

8. The laser diode according to claim 1, wherein the concave portion is
line-symmetrically formed with respect to a central axis in the width
direction of the current confinement structure.

8. The laser diode according to claim 1, wherein the concave portion
extends in a direction parallel to the extending direction of the current
confinement structure.

9. The laser diode according to claim 1, wherein the concave portion
extends in the direction intersecting the extending direction of the
current confinement structure.

10. The laser diode according to claim 1, wherein a plurality of the
concave portions are formed, and the concave portions are arranged in the
width direction of the current confinement structure and in the extending
direction of the current confinement structure.

11. The laser diode according to claim 10, wherein the concave portions
are nonuniformly arranged.

12. A laser diode comprising: an active layer; and a stacked structure
including one or a plurality of strip-shaped current confinement
structures confining a current which is injected into the active layer,
wherein the current confinement structure includes a pair of high
resistance regions with a current injection region in between, and
includes one or a plurality of strip-shaped concave portions extending in
an extending direction of the current injection region, the concave
portions disposed in a central part in a width direction of the current
injection region.

13. The laser diode according to claim 12, wherein a width of the concave
portion is 2 μm or above, and 5 μm or below.

14. The laser diode according to claim 12, wherein a plurality of the
concave portions are formed, and a space between the concave portions is
2 μm or above, and 5 μm or below.

15. The laser diode according to claim 12, wherein the concave portion is
line-symmetrically formed with respect to a central axis in the width
direction of the current injection region.

16. The laser diode according to claim 12, wherein the concave portion
extends in a direction parallel to the extending direction of the current
injection region.

17. The laser diode according to claim 12, wherein the concave portion
extends in the direction intersecting the extending direction of the
current injection region.

18. The laser diode according to claim 12, wherein two convex portions
are formed, and a space between the convex portions is uniform in the
extending direction of the current injection region.

19. The laser diode according to claim 12, wherein two concave portions
are formed, and a space between the concave portions in the extending
direction of the current injection region increases mainly towards an
emitting direction of light.

20. The laser diode according to claim 12, wherein two concave portions
are formed, and a space between the concave portions is uniform in the
extending direction of the current injection region, and a width of the
current injection region increases mainly towards the emitting direction
of light.

21. A laser diode comprising: an active layer; and a stacked structure
including one or a plurality of strip-shaped current confinement
structures confining a current which is injected into the active layer,
wherein the current confinement structure is in a ridge shape, and the
current confinement structure includes a contact layer having a width
smaller than the width of an upper part of the current confinement
structure, the contact layer disposed in an upper part of the ridge
shape.

22. The laser diode according to claim 21, wherein the stacked structure
includes one or a plurality of strip-shaped convex portions extending in
an extending direction of the current confinement structure.

23. The laser diode according to claim 21, wherein the current
confinement structure includes one or a plurality of strip-shaped concave
portions extending in the extending direction of the current confinement
structure, the concave portion disposed in the contact layer and in
positions immediately below the contact layer.

Description:

[0001] This application is a division of U.S. patent application Ser. No.
12/237,547, filed Sep. 25, 2008, the entirety of which is incorporated
herein by reference to the extent permitted by law. The present
application claims the benefit of priority to Japanese Patent Application
No. JP 2007-259429 filed in the Japanese Patent Office on Oct. 3, 2007,
the entirety of which is incorporated by reference herein to the extent
permitted by law.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a laser diode having one or a
plurality of current confinement structures, and in particular, relates
to a broad-area laser diode suitable for high output applications and a
method of manufacturing the same.

[0003] As a compact and low-cost laser light source with high reliability
and high output capability, a so-called broad-area laser diode including
a waveguide with its width (i.e. stripe width) widened has been utilized
in various fields such as displays, printing products, initialization
devices of optical disks, fabricating materials, and medical
applications. In a typical broad-area laser diode, a stripe width is at
least 5 μm or above. In many cases, the stripe width is 10 μm or
above, and several hundred μm as a maximum. As a technique related to
the broad-area laser diode, there is known a technique, for example,
described in Japanese Unexamined Patent Publication No. 2003-60288.

SUMMARY OF THE INVENTION

[0004] However, in the broad-area laser diode, the high-order mode may be
arbitrarily generated and the filament emission may further be produced.
Thus, control of NFP is difficult, and NFP with a stable and uniform
shape (top-hat shape) may hardly be obtained. Even in the case of NFP
with a stable and uniform shape under a certain condition, a change in
temperature, injection current, or electric power may cause that the NFP
is likely fluctuated and has a nonuniform shape.

[0005] Therefore, in the application in which the stable and uniform NFP
is desired, for example in the case where the broad-area laser diode is
applied to a display, there is a risk that the display quality is
degraded, because of generation of flicker, nonuniformity of brightness
on the screen, or the like.

[0006] In view of the forgoing, it is desirable to provide a laser diode
which enables NFP with a stable and uniform shape, and a method of
manufacturing the same.

[0007] According to an embodiment of the present invention, there is
provided a first laser diode having, on a semiconductor substrate, an
active layer, one or a plurality of strip-shaped current confinement
structures confining a current which is injected into the active layer,
and a stacked structure including one or a plurality of strip-shaped
convex portions extending in an extending direction of the current
confinement structure.

[0008] In the first laser diode according to an embodiment of the present
invention, one or a plurality of strip-shaped convex portions extending
in the extending direction of the current confinement structure are
provided within the stacked structure. Thereby, light emitted in the
active layer is guided by a waveguide structure corresponding to a
refractive index distribution formed by the current confinement structure
and the convex portion.

[0009] According to another embodiment of the present invention, there is
provided a second laser diode having an active layer, and a stacked
structure including one or a plurality of strip-shaped current
confinement structures confining a current which is injected into the
active layer. The current confinement structure is in a ridge shape, and
includes one or a plurality of strip-shaped concave portions extending in
an extending direction of the current confinement structure, the concave
portions disposed in an upper part of the current confinement structure
in the ridge shape.

[0010] In the second laser diode according to another embodiment of the
present invention, one or a plurality of strip-shaped concave portions
extending in the extending direction of the current confinement structure
are provided in the upper part of the current confinement structure in
the ridge shape. Thereby, light emitted in the active layer is guided by
a waveguide structure corresponding to a refractive index distribution
formed by the current confinement structure and the concave portion.

[0011] According to another embodiment of the present invention, there is
provided a third laser diode having an active layer, and a stacked
structure including one or a plurality of strip-shaped current
confinement structures confining a current which is injected into the
active layer. The current confinement structure includes a pair of high
resistance regions with a current injection region in between, and one or
a plurality of strip-shaped concave portions extending in an extending
direction of the current injection region, the concave portions disposed
in a central part in a width direction of the current injection region.

[0012] In the third laser diode according to another embodiment of the
present invention, the current confinement structure includes the pair of
high resistance regions with the current injection region in between, and
one or a plurality of strip-shaped concave portions extending in the
extending direction of the current injection region, the concave portions
disposed in the central part in the width direction of the current
injection region. Thereby, light emitted in the active layer is guided by
a waveguide structure corresponding to a refractive index distribution
formed by the current confinement structure and the concave portion.

[0013] According to another embodiment of the present invention, there is
provided a fourth laser diode having an active layer, and a stacked
structure including one or a plurality of strip-shaped current
confinement structures confining a current which is injected into the
active layer. The current confinement structure is in a ridge shape, and
the current confinement structure includes a contact layer having a width
smaller than the width of an upper part of the current confinement
structure, the contact layer disposed in an upper part of the ridge
shape.

[0014] In the fourth laser diode according to another embodiment of the
present invention, the contact layer having the width smaller than the
width of the upper part of the current confinement structure is provided
in the upper part of the current confinement structure in the ridge
shape. Thereby, light emitted in the active layer is guided by a
waveguide structure corresponding to a refractive index distribution
formed by the current confinement structure and the distribution of the
injection current via the contact layer.

[0015] A method of manufacturing a laser diode according to an embodiment
of the present invention includes steps of (A), (B), and (C) as follows.
The steps are:

(A) forming one or a plurality of strip-shaped concave portions on a
surface of a semiconductor substrate, (B) forming a first conductive
semiconductor layer, an active layer, and a second conductive
semiconductor layer in this order on the surface of the semiconductor
substrate on the concave portion side, the semiconductor substrate on
which the concave portion is formed, thereby forming a strip shaped
convex portion in an area corresponding to the concave portion in the
first conductive semiconductor layer on the active layer side, and (C)
forming a current confinement structure confining a current which is
injected into the active layer, on an upper part of the second conductive
semiconductor layer, the current confinement structure extending in an
extending direction of the concave portion.

[0016] In the method of manufacturing the laser diode according to an
embodiment of the present invention, one or a plurality of strip-shaped
convex portions extending in the extending direction of the current
confinement structure are formed in the first conductive semiconductor
layer by using the concave portion formed on the surface of the
semiconductor substrate. Thereby, light emitted in the active layer is
guided by a waveguide structure corresponding to a refractive index
distribution formed by the current confinement structure and the convex
portions.

[0017] According to the first laser diode and the second laser diode, and
a method of manufacturing a laser diode in embodiments of the present
invention, the light emitted in the active layer is guided by the
waveguide structure corresponding to the refractive index distribution
formed by the current confinement structure and the convex portion. Thus,
due to the interaction of the waveguide structure by both of the current
confinement structure and the convex portion, a transverse mode is
stabled and filament emission may be suppressed. Thereby, NFP with a
stable and uniform shape may be formed.

[0018] According to the third laser diode in an embodiment of the present
invention, the light emitted in the active layer is guided by the
waveguide structure corresponding to the refractive index distribution
formed by the current confinement structure and the concave portion.
Thus, due to the interaction of the waveguide structure by both of the
current confinement structure and the concave portion, a transverse mode
is stabled and filament emission may be suppressed. Thereby, NPF with a
stable and uniform shape may be formed.

[0019] According to the fourth laser diode in an embodiment of the present
invention, the light emitted in the active layer is guided by the
waveguide structure corresponding to the refractive index distribution
formed by the current confinement structure and the distribution of the
injection current via the contact layer. Thus, due to the interaction of
the waveguide structure by both of the current confinement structure and
the distribution of the injection current via the contact layer, a
transverse mode is stabled and filament emission may be suppressed.
Thereby, NPF with a stable and uniform shape may be formed.

[0020] Other and further objects, features and advantages of the invention
will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIGS. 1A and 1B are examples of a cross-sectional configuration
view of a laser diode according of a first embodiment of the present
invention.

[0022] FIG. 2 is an example of a top surface configuration view of the
laser diode of FIGS. 1A and 1B

[0023] FIG. 3 is another example of the top surface configuration view of
the laser diode of FIGS. 1A and 1B.

[0024] FIGS. 4A and 4B are cross sectional configuration views of a
modification of the laser diode of FIGS. 1A and 1B.

[0025] FIGS. 5A and 5B are examples of a cross-sectional configuration
view of a laser diode according to a second embodiment of the present
invention.

[0026]FIG. 6 is an example of a top surface configuration view of the
laser diode of FIGS. 5A and 5B.

[0027] FIG. 7 is another example of the top surface configuration view of
the laser diode of FIGS. 5A and 5B.

[0028] FIGS. 8A and 8B are cross-sectional configuration views of a
modification of the laser diode of FIGS. 5A and 5B.

[0029] FIGS. 9A and 9B are cross-sectional configuration views of another
modification of the laser diode of FIGS. 5A and 5B.

[0030] FIGS. 10A and 10B are cross-sectional configuration views of still
another modification of the laser diode of FIGS. 5A and 5B.

[0031] FIG. 11 is an example of a top surface configuration view of the
laser diode of FIGS. 10A and 10B.

[0032]FIG. 12 is another example of the top surface configuration view of
the laser diode of FIGS. 10A and 10B.

[0033] FIGS. 13A and 13B are examples of a cross-sectional configuration
view of a laser diode according to a third embodiment of the present
invention.

[0034]FIG. 14 is an example of a top surface configuration view of the
laser diode of FIGS. 13A and 13B.

[0035] FIG. 15 is another example of the top surface configuration view of
the laser diode of FIGS. 13A and 13B.

[0036] FIGS. 16A and 16B are cross-sectional configuration views of a
modification of the laser diode of FIGS. 13A and 13B.

[0037] FIGS. 17A and 17B are cross sectional configuration views of
another modification of the laser diode of FIGS. 13A and 13B.

[0038] FIG. 18 is an example of a top surface configuration view of the
laser diode of FIGS. 17A and 17B.

[0039] FIG. 19 is another example of the top surface configuration view of
the laser diode of FIGS. 17A and 17B.

[0040] FIGS. 20A and 20B are cross sectional configuration views of still
another modification of the laser diode of FIGS. 13A and 13B.

[0041]FIG. 21 is am example of a top surface configuration view of the
laser diode of FIGS. 20A and 20B.

[0042]FIG. 22 is another example of the top surface configuration view of
the laser diode of FIGS. 20A and 20B.

[0043]FIG. 23 is still another example of the top surface configuration
view of the laser diode of FIGS. 20A and 20B.

[0044]FIG. 24 is a top surface configuration view of still another
modification of the laser diode of FIGS. 13A and 13B.

[0045] FIGS. 25A and 25B are examples of a refractive index distribution
of a laser diode of the related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Preferred embodiments of the present invention will be described in
detail with reference to the accompanying drawings.

First Embodiment

[0047] FIG. 1A illustrates a cross-sectional configuration of a laser
diode 1 according to a first embodiment of the present invention. FIG. 1B
shows a refractive index distribution of the laser diode 1 of FIG. 1A.
FIG. 2 illustrates an example of a top surface configuration of the laser
diode 1 of FIG. 1A. In addition, FIG. 1A corresponds to a cross-sectional
configuration view taken along the direction of an arrow A-A in FIG. 2.
The laser diode 1 is an index guided laser including a semiconductor
substrate 10, a semiconductor layer 20 formed on the semiconductor
substrate 10 (stacked structure), and a ridge 29 formed in an upper part
of the semiconductor layer 20 (current confinement structure). The laser
diode 1 is also a broad-area laser in which a width WR of the ridge
29 is wide (at least 5 μm or above).

[0048] The semiconductor layer 20 has a configuration in which a lower
cladding layer 21, a first lower guide layer 22, a second lower guide
layer 23, an active layer 24, an upper guide layer 25, an upper cladding
layer 26, an intermediate layer 27, and a contact layer 28 are stacked in
this order from the semiconductor substrate 10 side. The ridge 29 is
provided in the upper part of the semiconductor layer 20. Specifically,
the ridge 29 is provided in the upper part of the upper cladding layer
26, and in the intermediate layer 27 and the contact layer 28, and has a
stripe convex shape extending in an emitting direction (axis direction)
of a laser light. Hereafter, the direction of the stacked layers
constituting the semiconductor layer 20 is referred to as a vertical
direction, and the direction perpendicular to the axis direction and the
vertical direction is referred to as a transverse direction.

[0049] The semiconductor substrate 10 is, for example, composed of n-type
GaAs, and is a patterned substrate having a concave portion 10A on its
surface on the semiconductor layer 20 side. In addition, the n-type
impurities are, for example, silicon (Si) and selenium (Se).

[0050] In an area corresponding to the ridge 29 (facing area), the concave
portion 10A extends in the extending direction of the ridge 29. When the
lower cladding layer 21, the first lower guide layer 22, the second lower
guide layer 23, and the active layer 24 are formed in this order on the
semiconductor substrate 10 through the use of the crystal growth method
in a manufacturing step (will be described later), the concave portion
10A preferably has a depth and a width which are large to the extent that
the active layer 24 is formed in a flat plane without concave and convex.
The concave portion 10 preferably has a depth D1 from 100 nm to 200
nm and a width W1 (width of the top of the aperture) from 5 μm to
10 μm. Also, as shown in FIG. 2, the concave portion 10A preferably
extends in the direction parallel to the extending direction (axis
direction) of the ridge 29. Alternatively, as shown in FIG. 3, in the
area facing the ridge 29, the concave portion 10A may extend in the
direction intersecting the extending direction (axis direction) of the
ridge 29. As shown in FIG. 2, the concave portion 10A is preferably
formed in the area facing the central part in a width direction
(transverse direction) of the ridge 29. Alternatively, the concave
portion 10A may be formed in the area except the area facing the central
part in the width direction (transverse direction) of the ridge 29.

[0051] The lower cladding layer 21 is, for example, composed of n-type
AlInP, and has a strip-shaped concave portion 21A which has a depth
smaller than the depth D1 of the concave portion 10A, and a width
smaller than the width W1 of the concave portion 10A, in the area
corresponding to the concave portion 10A of the semiconductor substrate
10. Due to the rib structure constructed by the concave portion 21A and a
convex portion 22A (will be described later), the concave portion 21A has
a depth D2 and a width W2 which produce a change on an
effective refractive index distribution in the transverse direction by
the ridge 29. The depth D2 is preferably from 50 nm to 100 nm and
the width W2 (width of the top of the aperture) is preferably from 5
μm to 10 μm.

[0052] When a semiconductor material is crystal-grown on the semiconductor
substrate 10 in a manufacturing step (will be described later), the
concave portion 21A is formed under the influence of the concave portion
10A. Thus, the depth D2 and the width W2 of the concave portion
21A may be set within a desired range by adjusting the thickness of the
lower cladding layer 21, or adjusting the depth D1 and the width
W1 of the concave portion 10A. In addition, in the case where the
concave portion 10A is set so as to have the depth D1 from 100 nm to
200 nm, and the width W1 from 5 μm to 10 μm, the lower
cladding layer 21 may have a thickness as the same size as that in the
case of the related art in which the semiconductor substrate 10 includes
no concave portion 10A.

[0053] As described above, because the concave portion 21A is formed under
the influence of the concave portion 10A, the concave portion 21A extends
in the direction parallel to the extending direction of the concave
portion 10A. Thus, as shown in FIG. 2, in the case where the concave
portion 10A is formed in the area facing the central part in the width
direction of the ridge 29, the concave portion 21A is also formed in the
area facing the central part in the width direction of the ridge 29. As
shown in FIG. 3, in the area facing the ridge 29, in the case where the
concave portion 10A extends in the direction intersecting the extending
direction of the ridge 29, the concave portion 21A also extends in the
direction intersecting the extending direction of the ridge 29.

[0054] The first lower guide layer 22 is, for example, composed of n-type
AlGaInP, and has a strip-shaped convex portion 22A which has a height
larger than the depth D1 of the concave portion 10A, and a width
smaller than the width W1 of the concave portion 10A, in the area
corresponding to the concave portion 10A of the semiconductor substrate
10. The convex portion 22A is formed so as to fill the concave portion
21A of the lower cladding layer 21. Thus, the convex portion 22A has a
convex shape projecting to the semiconductor substrate 10 side (side
opposite from the active layer 24), and extends in the direction parallel
to the extending direction of the concave portion 10A. Due to the rib
structure constructed by the concave portion 21A and the convex portion
22A, the convex portion 22A has the height (the depth D2) and the
width (the width W2) which produce a change on the effective
refractive index distribution in the transverse direction by the ridge
29. The height is preferably from 50 nm to 100 nm and the width (width of
the bottom of the convex) is preferably from 5 μm to 10 μm. In the
rib structure, the difference (refractive index difference) between the
refractive index on the lower cladding layer 21 side and the refractive
index on the first lower guide layer 22 side is preferably 0.1 or above.

[0055] The second lower guide layer 23 is, for example, composed of n-type
AlGaInP, and is formed in an almost-flat shape without concave and
convex, even in the position immediately above the convex portion 22A.

[0056] The active layer 24 is, for example, composed of undoped GaInP, and
is formed on the flat second lower guide layer 23 in a manufacturing step
(will be described later). Thus, the active layer 24 is also formed in an
almost-flat shape without concave and convex. In the active layer 24, the
area facing the ridge 29 is a light emitting region 24A. The light
emitting region 24A has a stripe width as the same size as that of the
bottom (the portion of the upper cladding layer 26) of the ridge 29
facing the light emitting region 24A. The light emitting region 24A is
coincident with a current injection region into which the current
confined in the ridge 29 is injected.

[0057] The upper guide layer 25 is, for example, composed of p-type
AlGaInP. Similarly to the active layer 24, the upper guide layer 25 is
formed in an almost-flat shape without concave and convex. In addition,
the p-type impurities are, for example, zinc (Zn), magnesium (Mg), and
beryllium (Be).

[0058] The upper cladding layer 26 is, for example, composed of p-type
AlInP. The intermediate layer 27 is, for example, composed of p-type
GaInP. The contact layer 28 is, for example, composed of p-type GaAs. As
described above, the ridge 29 is constructed by the upper part of the
upper cladding layer 26, and the intermediate layer 27 and the contact
layer 28. The contact layer 28 is formed over the whole upper surface of
the ridge 29.

[0059] In the laser diode 1, an upper electrode layer 31 is formed on the
upper surface of the ridge 29 (contact layer 28). An insulating layer 30
is formed over the side faces of the ridge 29, and the surface of the
upper cladding layer 26 except the area the ridge 29 is formed. In
addition, the upper electrode layer 31 may extend up to the surface of
the insulating layer 30. A lower electrode layer 32 is formed on a rear
surface (the surface opposite from the semiconductor layer 20 side) of
the semiconductor substrate 10.

[0060] The insulating layer 30 is, for example, composed of silicon oxide
(SiO2). The upper electrode layer 31 is, for example, formed by
stacking titanium (Ti), platinum (Pt), and gold (Au) in this order on the
contact layer 28, and is electrically connected to the contact layer 28.
The lower electrode layer 32 has, for example, a configuration formed by
stacking alloy of gold (Au) and germanium (Ge), nickel (Ni), and gold
(Au) in this order from the semiconductor substrate 10 side, and is
electrically connected to the semiconductor substrate 10.

[0061] The laser diode 1 having such a configuration may, for example, be
manufactured as will be described below.

[0062] First, the strip-shaped concave portion 10A having predetermined
width and depth is formed on the surface of the semiconductor substrate
10. Next, through the use of the crystal growth method, on the surface of
the semiconductor substrate 10 on the concave portion 10A side, the lower
cladding layer 21, the first lower guide layer 22, the second lower guide
layer 23, the active layer 24, the upper guide layer 25, the upper
cladding layer 26, the intermediate layer 27, and the contact layer 28
are stacked in this order from the semiconductor substrate 10 side.
Thereby, in the first lower guide layer 22 immediately below the active
layer 24, the strip-shaped convex portion 22A is formed in the area
(facing area) corresponding to the convex portion 10A, and the active
layer 24 is formed on the flat second lower guide layer 23. Next, for
example through the use of the dry etching method, the upper part of the
upper cladding layer 26, and the intermediate layer 27 and the contact
layer 28 are selectively etched. Thereby, the ridge 29 is formed in the
upper part of the semiconductor layer 20. After that, the insulating
layer 30, the upper electrode layer 31, and the lower electrode layer 32
are formed, and then a front end face S1 and a rear end face S2
are formed by cleaving the semiconductor substrate 10 in the direction
orthogonal to the extending direction of the ridge 29. In this manner,
the laser diode 1 according to the first embodiment is manufactured.

[0063] Next, the operation of the laser diode 1 according to the first
embodiment will be described. In the laser diode 1, when a predetermined
amount of voltage is applied between the lower electrode layer 32 and the
upper electrode layer 31, the current is confined by the ridge 29 and the
current is injected into the current injection region (light emitting
region 24A) of the active layer 24. Thereby, an electron and a hole are
recombined so that light emission is generated. This light is guided in
the semiconductor layer 20 by a waveguide structure corresponding to the
refractive index distribution formed by the ridge 29 and the convex
portion 22A. The light is also reflected by a pair of cleavage planes
(the front end face S1 and the rear end face S2) (refer to FIG.
2) facing each other in the extending direction (axis direction) of the
ridge 29. While the light moves back and forth between the pair of
cleavage planes, a laser oscillation is generated at a predetermined
wavelength, and thereby a laser beam is emitted outside from the cleavage
planes.

[0064] In a typical laser diode including the ridge structure, due to the
refractive index difference in the transverse direction by the ridge
structure, the light emitted in the light emitting region of the active
layer is confined from the transverse direction in a light waveguide. At
this time, in the case of the narrow-stripe laser diode in which the
ridge width is narrow (less than 5 μm), the refractive index
distribution in the transverse direction becomes the index guide type
(refer to FIG. 25A). However, in the case of the broad-area laser diode
in which the ridge width is wide, because of the plasma effect caused by
the current injected into the light emitting region, the refractive index
in the central part of the refractive index distribution in the
transverse direction formed by the ridge structure decreases, and the
central part of the refractive index distribution curves downwards as
shown in FIG. 25B. Thus, in this case, the refractive index distribution
in the transverse direction is not the index guide type in the whole
region. That is, in the central part, it is the gain guide type which may
be expressed as the anti-guide type. In this way, in the case where the
waveguide structure is the waveguide type (broad-area waveguide type)
which is the index guide type in combination with the gain guide type in
the central part, NFP is defined by the cooperative interaction of the
transverse mode formed by the index mode at both ends and the gain mode
in the central part. Therefore, it is important that the overall mode is
smoothly established while both of the modes are appropriately accorded
(refer to T. Asatsuma et al., Proceedings of SPIE, Vol. 61040C (2006)).

[0065] For example, there are known measures in which the refractive index
difference at both ends in the transverse direction of the ridge
structure is set within the predetermined range (for example,
approximately 0.005), and thereby the NFP approaches the top hat shape.
However, in the measures, it is difficult to sufficiently control the
gain region in the large central part, and thus the transverse mode may
be changed due to a change in injection current and temperature (refer to
D. Imanishi et al., Electronics Letters, 41 (2005) p 1172).

[0066] On the other hand, in the first embodiment, the strip-shaped convex
portion 22A extending in the extending direction of the ridge 29 is
provided in a light waveguide region (the second lower guide layer 23) of
the semiconductor layer 20. Thereby, as shown in FIG. 1B, because a
narrow-stripe index guide formed by the convex portion 22A is applied in
the gain region in the large central part, when the index guided
transverse mode is generated by the convex portion 22A, the transverse
mode is induced in the gain region. Then, the transverse mode generated
in the index region in the central part formed by the convex portion 22A
is combined with the transverse mode induced in the gain region so that
the index guide in the central part controls the transverse mode induced
in the gain region. When the transverse mode is generated in the index
region at both ends, the transverse mode in the index region is combined
with the transverse mode in the gain region so that the index guide at
both ends controls the width of the entire NFP to be within the
predetermined range. In this manner, the transverse mode generated in the
index region corresponding to the convex portion 22A, the transverse mode
generated in the gain region, and the transverse mode generated in the
index region at both ends are cooperatively combined with each other.
Thereby, the transverse mode generated in the gain region loses
controllability and becomes easy to be controlled. As a result, the NFP
with the stable and uniform shape may be formed as a whole.

[0067] As shown in FIG. 2, in the case where the convex portion 22A is
formed in the area facing the central part in the width direction
(transverse direction) of the ridge 29, the stripe in an index guide
induction mode, and the stripe in an gain guide induction mode are
parallel to each other so that there is an advantage that these modes are
likely and smoothly combined. As shown in FIG. 3, in the area facing the
ridge 29, in the case where the convex portion 22A extends in the
direction intersecting the extending direction (axis direction) of the
ridge 29, formation of a peak of a certain spatial periodicity is
suppressed within the whole transverse mode, and thus the top-hat shape
is easily formed as a whole since various modes are excited at the same
time.

Modification of First Embodiment

[0068] In the first embodiment, although only one convex portion 22A (the
concave portion 21A and the concave portion 10A) is formed, two convex
portions 22A may be formed in the central part, for example, as shown in
FIG. 4A or three or more convex portions 22A may be formed. In the case
where the two convex portions 22A (the concave portions 21A and the
concave portions 10A) are formed in the central part, the refractive
index distribution is, for example, as shown in FIG. 4B.

[0069] In the first embodiment, although the convex portion 22A is
projected to the semiconductor substrate 10 side, the convex portion,
instead of the concave portion 10A, may be provided on the semiconductor
substrate 10, and the concave portion may be provided corresponding to
the convex portion, on the interface of the first lower guide layer 22 on
the semiconductor substrate 10 side. In this case, on the interface of
the lower cladding layer 21 on the first lower guide layer 22 side, the
convex portion which is projected to the active layer 24 side may be
formed corresponding to the concave portion of the first lower guide
layer 22.

Second Embodiment

[0070] FIG. 5A illustrates an example of the cross-sectional configuration
of a laser diode 2 according to a second embodiment of the present
invention. FIG. 5B shows the refractive index distribution of the laser
diode 2 of FIG. 5A. FIG. 6 shows an example of a top surface
configuration of the laser diode 2 of FIG. 5A. In addition, FIG. 5A
corresponds to the cross-sectional configuration view as viewed from the
direction of an arrow A-A of FIG. 6. Similarly to the first embodiment,
the laser diode 2 is an index guided laser including a semiconductor
substrate 10, a semiconductor layer 20 (stacked structure) formed on the
semiconductor substrate 10, and a ridge 29 (current confinement
structure) formed in the upper part of the semiconductor layer 20. The
laser diode 2 is also a broad-area laser in which a width WR of the
ridge 29 is wide (at least 5 μm or above).

[0071] The laser diode 2 has a configuration different from that of the
first embodiment in that a pair of concave portions 33 are provided in
the ridge 29, instead of providing the convex portion 22A, the concave
portion 21A and the concave portion 10A. Thus, hereafter, the difference
from the first embodiment will be described in detail, and the
description common to the first embodiment will be appropriately omitted.

[0072] In the second embodiment, as described above, the pair of concave
portions 33 are formed in the ridge 29, and an upper part of the ridge 29
is separated into three portions. Hereafter, in the separated three
portions, the portions located at both ends in the width direction of the
ridge 29 are referred to as a ridge portion 29A and a ridge portion 29C,
and the portion sandwiched between the ridge portions 29A and 29C are
referred to as a ridge portion 29B.

[0073] The concave portions 33 are grooves reaching a predetermined depth
from the upper surface of the ridge 29. The concave portions 33 extend in
the extending direction of the ridge 29. Thus, the ridge portions 29A,
29B, and 29C also extend in the extending direction of the ridge 29. A
depth D3 of the concave portion 33 and a width W3 in the
transverse direction of the concave portion 33 may be arbitrarily set, as
long as the depth D3 and the width W3 are within the range so
that the ridge portions 29A, 29B, and 29C form a waveguide structure as a
whole. For example, the depth D3 is preferably equal to or smaller
than a height D4 of the ridge 29. The width W3 is preferably
from 2 μm to 5 μm.

[0074] As shown in FIGS. 5A and 6, the concave portions 33 preferably
extend in the direction parallel to the extending direction of the ridge
29. Alternatively, as shown in FIG. 7, the concave portions 33 may extend
in the direction intersecting the extending direction of the ridge 29. As
shown in FIG. 5A, the concave portions 33 may be formed in the central
part in the width direction of the ridge 29. Alternatively, the concave
portions 33 may be formed in the area except the central part in the
width direction of the ridge 29. For example, as shown in FIG. 8A, the
concave portions 33 may be formed in the positions where the ridge 29 is
equally separated into three portions in the width direction.
Alternatively, for example, as shown in FIG. 9A, the concave portions 33
may also be formed in the vicinity of both ends in the width direction of
the ridge 29. FIG. 8B and FIG. 9B show the refractive index distributions
of the laser diodes of FIG. 8A and FIG. 9A, respectively.

[0075] The ridge 29 is separated into the ridge portion 29A, 29B, and 29C
by the concave portions 33, and a confined current is injected into the
active layer 24 from the ridge portions 29A, 29B, and 29C. An upper
electrode layer 31 is provided on upper surfaces of the ridge portions
29A, 29B, and 29C, and the upper surfaces of the ridge portions 29A, 29B,
and 29C are electrically connected via the upper electrode layer 31.
Thus, it may be said that, as a circuit, the ridge portions 29A, 29B, and
29C are connected in parallel between the upper electrode layer 31 and
the lower electrode layer 32. However, as exemplified above, because the
ridge 29 is separated into the ridge portions 29A, 29B, and 29C only by
the concave portions 33 with the small depth D3 and the small width
W3, the current injection regions of the ridge portions 29A, 29B,
and 29C are partially overlapped with each other. As a result, one light
emitting region 24A is constituted. Therefore, the ridge portions 29A,
29B, and 29C form a waveguide structure as a whole in the semiconductor
layer 20.

[0076] For example, as shown in FIG. 5A, in the case where the concave
portions 33 are formed in the central part in the width direction of the
ridge 29, a width W5 of the ridge portion 29B formed in the central
part of the ridge 29 is a with (less than 5 μm) as the same size as
the stripe width of the narrow-stripe laser diode, and a width W4
and a width W6 of the ridge portions 29A and 29C formed in the area
except the central part of the ridge 29 are widths (at least 5 μm or
above) as the same size as the stripe width of the broad-area laser
diode. Thereby, as shown in FIG. 5B, in the center of the broad-area
waveguide of a width WR formed between the outer end of the ridge
portion 29A and the outer end of the ridge portion 29C, applied is a
narrow-stripe index guide formed by the ridge portion 29B. Thus, the
transverse mode generated in the index region in the central part formed
by the ridge portion 29B is combined with the transverse mode generated
in the gain region included in the broad-area waveguide of the width
WR so that the index guide in the central part controls the
transverse mode generated in the gain region. When the transverse mode is
generated in the index region at both ends, the transverse mode in the
index region is combined with the transverse mode in the gain region, and
thus the index guide at both ends controls the width of the entire NFP to
be within the predetermined range. In this manner, the transverse modes
generated in the ridge portions 29A, 29B, and 29C are cooperatively
combined with each other. Thereby, the transverse mode generated in the
gain region loses controllability and becomes easy to be controlled. As a
result, the NFP with the stable and uniform shape may be formed as a
whole.

[0077] For example, as shown in FIG. 8A, in the case where the concave
portions 33 are formed in positions where the ridge 29 is equally
separated into the three portions in the width direction, the widths
W4, W5, and W6 of the equally-separated ridge portions
29A, 29B, and 29C become widths (approximately from 10 μm to 30 μm)
as the same size as the stripe width of the broad-area laser diode having
a relatively small width. Thereby, as shown in FIG. 8B, the broad-area
index guide which has a relatively small width is formed corresponding to
each of the ridge portions 29A, 29B, and 29C. Thus, the transverse modes
formed by the ridge portions 29A, 29B, and 29C are combined with each
other, and the NFP with the stable and uniform shape may be formed as a
whole.

[0078] In addition, as described above, the widths W4, W5, and
W6 of the ridge portions 29A, 29B, and 29C, respectively, may be
disposed with unequal intervals in between. For example, the ridge
portion 29B may be formed with a relatively small width and a plurality
of pitches of the transverse mode are applied. Thus, by the interferences
(beat cycle) of these modes, the NFP with the stable and uniform shape
may be formed as a whole.

[0079] For example, as shown in FIG. 9A, in the case where the concave
portions 33 are formed in the vicinity of both ends in the width
direction of the ridge 29, the width W5 of the ridge portion 29B
formed in the central part of the ridge 29 becomes the width (at least 5
μm or above) as the same size as the stripe width of the broad-area
laser diode, and the widths W4 and W6 of the ridge portions 29A
and 29C formed in the area except the central part of the ridge 29 become
the widths (less than 5 μm) as the same size as the stripe width of
the narrow-stripe laser diode.

[0080] Here, in the typical broad-area laser diode including the index
guide structure by the ridge, the transverse mode in the index region at
both ends in the width direction of the ridge tends to oscillate prior to
the transverse mode in the gain region in the central part, and it is
likely that, in the beginning of the oscillation (when the amount of the
injection current is still small), a double-angle mode in which the
transverse mode at both ends of the index guide region is prioritized is
applied. Also, in the case where the index guide structure is formed by
the single ridge, because the effective width of the index guide changes
according to the intensity of the plasma effect in the central part so
that, by the change of the effective width, the accordance of the index
mode at the bodes ends and the gain mode in the central part is shifted
and a large surge in the mode may be generated according to the amount of
the injection current.

[0081] However, in the second embodiment, because the concave portions 33
are formed in the vicinity of both ends in the width direction of the
ridge 29 and the narrow-stripe ridge portions 29A and 29C are provided at
both ends of the broad-area ridge portion 29B, the current is actively
injected into the gain region in the central part from the broad-area
ridge portion 29B, and the amount of current injected into the index
regions at both ends from the narrow-stripe ridge portions 29A and 29C is
reduced. Thus, the intensity of the transverse mode in the gain region in
the central part, and the intensity of the transverse mode in the index
region at both ends may be in good balance. Thereby, the combination
between the both modes is smoothly performed, and this enables that the
transverse mode in the index region at both ends is suppressed from being
distorted, and the gain in the central part increases. Also, by the index
guide at both ends, the width of the entire NFP may be controlled within
a predetermined range. As a result, the NFP with the stable and uniform
shape may be formed as a whole.

Modification of Second Embodiment

[0082] In the second embodiment, the concave portions 33 are formed in the
vicinity of both ends in the width direction of the ridge 29, and the
narrow-stripe ridge portions 29A and 29C are provided at both ends of the
broad-area ridge portion 29B. Thereby, the intensity of the transverse
mode in the gain region in the central part and the intensity of the
transverse mode in the index region at both ends are in good balance.
However, as shown in a laser diode 3 in FIG. 10A, instead of the concave
portions 33, a strip-shaped electrode (an upper electrode layer 31) may
be provided only on the upper surface of the central part where the ridge
portion 29B of the ridge 29 is provided. The refractive index
distribution of the laser diode 3 of FIG. 10A is shown in FIG. 10B. Even
in this case, the current is actively injected into the gain region in
the central part, from the upper electrode layer 31, and the amount of
the current injected into the index region at both ends may be reduced.
As a result, the NFP with the stable and uniform shape may be formed as a
whole.

[0083] In the modification of the second embodiment, as shown in FIG. 11
(an example of the top surface configuration of FIG. 10A), the electrode
may extend in parallel to the extending direction of the ridge 29.
Alternatively, as shown in FIG. 12, (another example of the top surface
configuration of FIG. 10A), the electrode may extend in the direction
intersecting the extending direction of the ridge 29.

Third Embodiment

[0084] FIG. 13A illustrates a cross-sectional configuration of a laser
diode 4 according to a third embodiment of the present invention. FIG.
13B shows a refractive index distribution of the laser diode 4 of FIG.
13A. FIG. 14 shows an example of a top surface configuration of the laser
diode 4 of FIG. 13A. In addition, FIG. 13A corresponds to the
cross-sectional configuration view as viewed from the direction of an
arrow A-A of FIG. 14. The laser diode 4 is a gain guided laser including,
instead of the ridge 29 in the first embodiment, a strip-shaped upper
electrode layer 34 (current injection region), and a pair of insulating
layers 35 (high resistance regions) sandwiching the upper electrode layer
34 from the width direction of the upper electrode layer 34, the upper
electrode layer 34 and the pair of insulating layers 35 disposed in the
upper part of a semiconductor layer 20 (stacked structure). The laser
diode 4 is also a broad-area laser in which a width WS of the upper
electrode layer 34 is wide (at least 5 μm or above).

[0085] The upper electrode layer 34 is, for example, formed by stacking
titanium (Ti), platinum (Pt), and gold (Au) in this order on a contact
layer 28, and is electrically connected to the contact layer 28. The
upper electrode layer 34 is formed in a stripe and flat shape extending
in an emitting direction (axis direction) of a laser light, and form a
current confinement structure together with the pair of insulating layers
35. The insulating layers 35 are, for example, composed of silicon oxide
(SiO2).

[0086] In the third embodiment, in an area (facing area) corresponding to
the upper electrode layer 34, the concave portion 10A extends in the
extending direction of the upper electrode layer 34. As shown in FIG. 14,
the concave portion 10A preferably extends in the direction parallel to
the extending direction (axis direction) of the upper electrode layer 34.
Alternatively, as shown in FIG. 15, in the area facing the upper
electrode layer 34, the concave portion 10A may extend in the direction
intersecting the extending direction of the upper electrode layer 34. As
shown in FIG. 14, the concave portion 10A is preferably formed in the
area facing the central part in a width direction (transverse direction)
of the upper electrode layer 34. Alternatively, the concave portion 10A
may be formed in the area except the area facing the central part in the
width direction (transverse direction) of the upper electrode layer 34.

[0087] Due to the rib structure constructed by a concave portion 21A and a
convex portion 22A, the concave portion 21A of a lower cladding layer 21
has a depth D2 and a width W2 which produce a change on an
effective refractive index distribution in the transverse direction by
the current confinement structure. The depth D2 is preferably from
50 nm to 100 nm, and the width W1 (width of the aperture) is
preferably from 5 μm to 10 μm.

[0088] As shown in FIG. 14, in the case where the concave portion 10A is
formed in the area facing the central part in the width direction of the
upper electrode layer 34, the concave portion 21A is also formed in the
area facing the central part in the width direction of the upper
electrode layer 34. As shown in FIG. 15, in the area facing the upper
electrode layer 34, in the case where the concave portion 10A extends in
the direction intersecting the extending direction of the upper electrode
layer 34, the concave portion 21A also extends in the direction
intersecting the extending direction of the upper electrode layer 34.

[0089] Due to the rib structure constructed by the concave portion 21A and
the convex portion 22A, the convex portion 22A of the first lower guide
layer 22 has a height (the depth D2) and a width (the width W2)
which produce a change on the effective refractive index distribution in
the transverse direction by the current confinement structure. The height
is preferably from 50 nm to 100 nm, and the width (width of the bottom of
the convex) is preferably from 5 μm to 10 μm. In the rib structure,
the difference (refractive index difference) between the refractive index
on the lower cladding layer 21 side and the refractive index on the first
lower guide layer 22 side is preferably 0.1 or above.

[0090] In the active layer 24, the area facing the upper electrode layer
34 is a light emitting region 24A. The light emitting region 24A has a
stripe width as the same size as that of the upper electrode layer 34
facing the light emitting region 24A. The light emitting region 24A is
coincident with a current injection region into which the current
confined by the current confinement structure is injected.

[0091] Each of an upper cladding layer 26, an intermediate layer 27, and
the contact layer 28 is formed in a flat shape without concave and
convex. On the flat surface of the contact layer 28, the upper electrode
layer 34 and the pair of insulating layers 35 are formed. The upper
electrode layer 34 may extend onto the surface of the insulating layers
35. However, in this case, a portion of the upper electrode layer 34
which is in contact with the contact layer 28 construct the current
confinement structure together with the insulating layers 35.

[0092] In the laser diode 4 according to the third embodiment, when a
predetermined amount of voltage is applied between the lower electrode
layer 32 and the upper electrode layer 34, the current is confined by the
current confinement structure (hereafter, simply referred to as the
current confinement structure) constructed by the upper electrode layer
34 and the pair of insulating layers 35, and the current is injected into
the current injection region (light emitting region 24A) of the active
layer 24. Thereby, an electron and a hole are recombined so that light
emitting is generated. This light is guided in the semiconductor layer 20
by a waveguide structure corresponding to the refractive index
distribution formed by the current confinement structure and the convex
portion 22A. The light is also reflected by a pair of cleavage planes (a
front end face S1 and a rear end face S2) (refer to FIG. 14)
facing each other in the extending direction (axis direction) of the
current confinement structure. While the light moves back and forth
between the pair of cleavage planes, a laser oscillation is generated at
a predetermined wavelength, and thereby a laser beam is emitted outside
from the cleavage planes.

[0093] In a typical laser diode including the gain guided structure, due
to the refractive index difference in the transverse direction by the
current confinement structure, the light emitted in the light emitting
region of the active layer is confined from the transverse direction in a
light waveguide. At this time, in the case where an amount of current
within the range from the oscillation threshold value current up to twice
the oscillation threshold value current is injected into the active
layer, an hermite-gaussian mode is exhibited in sequence from a low-order
mode to be overlapped (refer to T. Asamatsu et al., Proceedings of SPIE,
Vol. 6104, 61040C (2006)). In this case, the uniformity of the transverse
mode is superior in comparison with the case of the index guide type, and
the NFP is close to the top-hat shape. However, when an amount of current
over twice the oscillation threshold value current is injected into the
active layer, the transverse mode is disordered, and the NFP is in an
incomplete top-hat shape. Further, filament emission may be generated and
the mode fluctuation may be produced.

[0094] On the other hand, in the third embodiment, the strip-shaped convex
portion 22A extending in the extending direction of the current
confinement structure is provided in the light waveguide region (the
second lower guide layer 23) of the semiconductor layer 20. Thereby, as
shown in FIG. 13B, because a narrow-stripe index guide formed by the
convex portion 22A is applied in a part of the large gain region, when
the index guided transverse mode is generated by the convex portion 22A,
the transverse mode is induced in the gain region. Then, the transverse
mode generated in the index region in the central part formed by the
convex portion 22A is combined with the transverse mode induced in the
gain region so that the index guide controls the transverse mode induced
in the gain region.

[0095] Because the transverse mode is gain guided as a whole, the
hermite-gaussian mode is exhibited in sequence from the low-order mode to
be overlapped. However, the position to be a "seed" of the transverse
mode is fixed by the index guide formed by the convex portion 22A. Thus,
the transverse mode is relatively stable, and disorder of the transverse
mode and fluctuation of the mode are suppressed. Even in the case where
an amount of current over twice the oscillation threshold value current
is injected into the active layer, the transverse mode with the ordered
gain guide may be maintained. As a result, the NFP with the stable and
uniform shape may be formed as a whole.

[0096] As shown in FIG. 14, in the case where the convex portion 22A is
formed in the area facing the central part in the width direction
(transverse direction) of the upper electrode layer 34, the stripe of an
index guide induction mode and the stripe of a gain guide induction mode
become parallel with each other so that there is an advantage that these
modes are likely and smoothly combined. As shown in FIG. 15, in the area
facing the upper electrode layer 34, in the case where the convex portion
22A extends in the direction intersecting the extending direction (axis
direction) of the upper electrode layer 34, formation of a peak of a
certain spatial periodicity is suppressed within the whole transverse
mode, and thus the top-hat shape is easily formed as a whole since
various modes are excited at the same time.

Modification of Third Embodiment

[0097] In the third embodiment, although one convex portion 22A (the
concave portion 21A and the concave portion 10A) is formed, two or more
convex portions 22A may be formed. For example, in the case where the two
convex portions 22A are provided as shown in FIG. 16A, a space Wi between
the two convex portions 22A may have a width larger than a width (a width
of a portion of an upper electrode layer 34 which is in contact with a
contact layer 28) of an upper electrode layer 34. FIG. 16B shows a
refractive index distribution of the laser diode of FIG. 16A.

[0098] In this case, the width of the transverse mode is defined by the
space Wi, and the width of the transverse mode is suppressed from being
larger than the space Wi. For example, in the case where the two convex
portions 22A are provided, the width of the space Wi between the two
convex portions 22A may be almost equal to the width (a width of a
portion of the upper electrode layer 34 in contact with the contact layer
28) of the upper electrode layer 34. In this case, the transverse mode
close to the ridge-shaped index guide may be formed. For example, in the
case where the two convex portions 22A are provided, the space Wi may be
smaller than the width (a width of a portion of the upper electrode layer
34 in contact with the contact layer 28) of the upper electrode layer 34.
In this case, the two index guides control the transverse mode induced in
the gain region. In this manner, because both of the transverse modes are
cooperatively combined with each other, the transverse mode generated in
the gain region loses controllability and becomes easy to be controlled.
As a result, the NFP with the stable and uniform shape may be formed as a
whole.

[0099] In the third embodiment, although the convex portion 22A, the
concave portion 21A, and the concave portion 10A are provided, similarly
to the second embodiment, instead of providing the convex portion 22A,
the concave portion 21A, and the concave portion 10A, a concave portion
33 may also be provided immediately below the upper electrode layer 34 as
shown in a laser diode 5 of FIGS. 17A and 18 (or FIG. 19). FIG. 17B shows
the refractive index distribution of the laser diode of FIG. 17A. In this
case, as shown in FIG. 17B, because the gain mode is fixed by the
anti-guiding effect in which the gain guide mode is formed by one convex
portion 33, the transverse mode generated in the gain region loses
controllability and becomes easy to be controlled. As a result, the NFP
with the stable and uniform shape may be formed as a whole.

[0100] In the third embodiment, instead of providing the convex portion
22A, the concave portion 21A, and the concave portion 10A, two concave
portions 33 may be provided immediately below the upper electrode layer
34 as shown in FIG. 20A. FIG. 20B shows the refractive index distribution
of the laser diode of FIG. 20A. In this case, as shown in FIG. 20B, a
narrow-stripe index guide formed by the ridge shape sandwiched between
the two concave portions 33 is applied in the gain region in the large
central part. Thus, when the index guided transverse mode is generated by
the ridge shape, the transverse mode is induced in the gain region. Then,
the transverse mode generated in the index region in the central part
formed by the ridge shape, and the transverse mode induced in the gain
region are combined with each other so that the ridge-shaped index guide
controls the transverse mode induced in the gain region. In this manner,
because both of the transverse modes are cooperatively combined with each
other, the transverse mode generated in the gain region loses
controllability and becomes easy to be controlled. As a result, the NFP
with the stable and uniform shape may be formed as a whole.

[0101] However, in this case, as shown in FIG. 21, the concave portions 33
preferably extend in the direction parallel to the extending direction
(axis direction) of the upper electrode layer 34. As shown in FIG. 22, in
the area facing the upper electrode layer 34, the width of the upper
electrode layer 34 may be decreased toward a rear end face S2 from a
front end face S1. As shown in FIG. 23, in the area facing the upper
electrode layer 34, a space between the two concave portions 33 may be
decreased toward the rear end face S2 from the front end face S1. In both
of the cases of FIGS. 22 and 23, formation of the transverse mode is
modulated in a resonator length direction so that there is no risk that
light intensity in only a specific region increases as the amount of
current increases. As a result, the spatial hole-burning and filament
emission may be suppressed.

[0102] In the third embodiment, as shown in FIG. 24, instead of providing
the convex portion 22A, the concave portion 21A, and the convex portion
10A, a plurality of concave portions 33 immediately below the upper
electrode layer 34 may be arranged in the extending direction of the
upper electrode layer 34 and in the width direction of the upper
electrode layer 34 as well. In this case, the concave portions 33 may be
uniformly arranged in two dimensions, or may be nonuniformly arranged in
two dimensions.

[0103] Hereinbefore, although the present invention is described with the
embodiments and the modifications, the present invention is not limited
to these and various modifications are available.

[0104] For example, in the embodiments, the present invention is described
with an example of the laser diode of AlGaInP type compound. However, the
present invention is also applicable to a laser diode of other compounds,
for example, a red laser diode such as AlInP type and GaInAsP type, a
laser diode of gallium nitride such as GaInNtype and AlGaInN type and a
laser diode of II-VI group such as ZnCdMgSSeTe. The present invention is
also applicable to a laser diode in which the oscillation wavelength is
not limited to a visible range, such as AlGaAs type, InGaAs type, InP
type, and GaInAsNP type.

[0105] It should be understood by those skilled in the art that various
modifications, combinations, sub-combinations and alterations may occur
depending on design requirements and other factors insofar as they are
within the scope of the appended claims or the equivalents thereof.